The invention relates to improvements in method and apparatus for the production of a free flowing coal suspension high in solids content involving the comminuting of raw or processed coal or coal without prior thermal drying by the suspension of the particles in water and the addition of viscosity reducing additives.
In the handling of coal, aqueous coal suspension high in solids content can, like oil, be transported in pipelines in tankers, can be stored in tanks, and can be directly burned in power plants or other industrial firings or serve as charging stock for coal gasification.
Such coal suspensions offer various advantages with respect to environmental protection. In the matter of a railroad accident involving tank cars, for example, the environmental risk is reduced when the vehicle is loaded with a coal suspension instead of heating oil. When a coal suspension is produced from a low sulfur and low ash coal or from a correspondingly processed coal, then the environmental pollution is also correspondingly low. Coal suspensions enable pipeline transport over substantial distances without dewatering of the coal being necessary at the location where the coal is used. Coal transport by this means is praticularly advantageous in regions lacking the capability of other forms of transport.
In instances where transport is made of coal suspensions, the demands to be made of such suspension depend on the properties of coal to be utilized, and the type of transport provided, and on the intended employment of the suspension. In general, the properties of the coal can be designated by the following particulars:
the solids content, usually 60, frequently 65% by mass percentage; PA1 the maximum grain size, usually 0.2 or 0.3 mm; PA1 the overs of a screen, limited in the coarse range, for example, a maximum of 10% coarser than 0.09 mm or a maximum of 30% coarser than 0.07 mm; PA1 the fine-grained component, usually slightly under 0.001 mm; PA1 an adequately low and chronologically constant viscosity given a corresponding shear rate (essential when used for pipeline transport over longer distances); PA1 an adequate stability during the intended transport and storing duration (i.e., no de-mixing based on grain size and solids concentration, particularly no formation of solid sediment).
The ignition and burn-out behavior of the coal particles and the demand to avoid unnecessary grinding costs result in that a grain size distribution, represented in the RRSB diagram (DIN 66145) should have approximately the shape of a backward "S" lying obliquely.
A so-called bimodel distribution with a mixing gap between 0.02 and 0.045 mm is recommended by Ferrini (Ferrini, F., et al) "Optimization of Particle Grading for High Concentration Coal Slurry", Preprint 132, 9th International Conference on Hydraulic Transport of Solids in Pipes, Rome, October 1984.
In accord with patent application, German No. 32 48 550 A1, a suitable size distribution of a coal suspension having excellent flowability is characterized in that the coal powder is 71 to 85% of mass coal particles having particle sizes of 74 .mu.m or smaller. Further, over and above this, the requirement involves a particle size distribution that, given graphic representation of this particle size distribution in a "Rosin-Rammler Diagram", the slope of the straight line which connects two points--of which one corresponds to the quantity (in mass percent) of particles having particles below 44 .mu.m and the other corresponds to the quantity (in mass percent) of particles having particle sizes below 74 .mu.m--exhibits a value of 0.4 through 0.9 expressed as a value of tg .alpha..
Mutli-stage comminution and grading operations are unavoidable when transferring the above described scientific perceptions into large scale industrial practice. In order to be able to establish one of the aforementioned grain size distributions, the following operations are, for example, required in prior art practice. That is, a first grinding in a closed circulation, i.e., with a grading device upon return of the coarse product to a grain size below 0.3 mm, a second grading device then removes the component between 0.3 and 0.045 mm from the fine product as a first finished product component, and this is followed by a second grinding circulation with a further grading means in order to regrind the component finer than 0.045 mm to finer than 0.02 mm.
With respect to apparatus employed, ball mills and an air sifter operating dry after previous drying are suitable, whereby the drying and the first grinding stage can be combined as grinding and drying in an appropriately equipped system. However, it is not feasible energy-wise to first dry the coal in order to then in turn mix it with water to form a suspension.
Wet grinding with wet ball mills and with hydrocyclones (with super-fine wet screening as well as given coarse separating cuts) is also possible. In the grading, however, the solids content of the fine product is limited to under 40% mass so that further costs arise when the product must be concentrated to a solids content of 60 to 80% mass by means of thickeners, filters, centrifuges and the like. It is also known (R. Klimpel, Slurry Rheology Influence on the Performance of Mineral/Coal Grinding Circuits, Mining Engineering Reprints, Dec. 1982 and Jan. 1983) to grind mineral raw materials given high solids contents with the addition of viscosity reducing reagents. The report describes the effect of reagents but provides no inducements as to how coal is to be mechanically treated in order to produce suspensions that are usable on a large industrial scale. The required grain size distribution, and particularly how it could be accomplished, are not discussed.
An object of the present invention is to obtain an economical method and apparatus for the production of a coal suspension for large scale industrial operations which does not exhibit the aforementioned disadvantages and deficiencies.
A feature of the invention in achieving the above object is accomplished in that at least a sub-quantity of the viscosity reducing additives are added to the coal before grinding given a solids content which is greater than or equal to the required solids content of the finished suspension and grinding is carried out in at least two stages with a mill having a calibrating effect being used in at least one grinding stage and a mill without calibrating effect is used in at least one grinding stage.
What is to be understood by a mill with a calibrating effect is a mill wherein either all grinding stock must pass an opening of a predefined size when it departs the grinding space such as, for example, a fine gyratory crusher, a sieve hammer pulverizer, sieve ball mills, or the structural type of operating mode of a mill which has the effect that the coarsest particles of the grinding stock are always primarily comminuted. This is especially the case with drum and vibration grinding mills in which rods serve as grinding bodies. Coarse particles are held by the rods at a distance so that the finer particles are not crushed by the rods.
A further object of the invention is to provide an improved method and apparatus for the production of a coal suspension which achieves the production of coal having the capabilities of fluid transport by more efficient and economical means than heretofore available.
An important feature of the invention is comprised in a combination of steps including at least two grinding stages having specific mills and no separate grading devices and no return is required. Also important is the addition of viscosity reducing agents before grinding. In accordance with the features of the invention, efficiency and economy is accomplished which is not anticipated.
Other objects, advantages and features will become more apparent with the disclosre of the principles of the invention in connection with the description of the preferred embodiment in the specification, claims and drawings, in which: